Mechanics 1條

網路上的路人甲

2011-05-08 7:05 pm

A bullet of mass 0.01kg travelling at 100 m s-1 is stopped by 0.1m of concrete. What retarding force is applied to the bullet by the concrete?
網上都多人都咁求

To find out the magnitude of force, we consider the equation

F = ma
So, we need to find out the value of m and a.
First, m is given, which is 0.01 kg.
Using formulas for uniform acceleration,

v2-u2=2as, together with the given data,

u = 100, s = 0.1, v = 0

we have

02-1002=2a(0.1)

a = -50000
Retarding force

= (0.01)(50000)

= 500N而我咁求

LOSE KE = W.D. AGANIST FRICTION

1/2M U^2 = F X S

1/2 (0.01) (100)^2 = 0.01 X F

F=500N

其實題目冇講到粒子彈射到入塊土幾深 ,佢只係話射左入塊0.1M 厚既土到就停左,而我咁樣做得唔得? THZ

更新1:

其實我係咪應寫成 1/2MV^2 -1/2MU^2 = -WD AGAINST FRICTION 先岩? 之後左右約左個負號再寫1/2M U^2 = F X S 1/2 (0.01) (100)^2 = 0.01 X F F=500N

回答 (1)

天同

2011-05-08 8:49 pm

✔ 最佳答案

The use of the "work done-energy" method is ok.
In fact, the "work done-energy" method is just the result of combining the two-step solution given by the use of F=ma and v^2=u^2 + 2as.

Q: 其實題目冇講到粒子彈射到入塊土幾深 ,佢只係話射左入塊0.1M 厚既土到就停左,而我咁樣做得唔得?

This is only a mater of language. The question says, ".... is stopped by 0.1m of concrete,...". It already indicates that the bullet is brought to rest in 0.1 m in the concrete.

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